Reduced coenzyme Q10 is superior to oxidized coenzyme Q10 in terms of oral absorbability, and thus it is a compound that is very useful as an antioxidant. For example, reduced coenzyme Q10 can be obtained by carrying out a reduction reaction of oxidized coenzyme Q10 obtained by a conventionally known method comprising synthesis, fermentation, extraction from a natural product, or the like (Patent Literature 1).
Known examples of substances (reducing agents) effective for reduction of oxidized coenzyme Q10 to reduced coenzyme Q10 include sodium borohydride (Non-Patent Literature 1), sodium dithionite (Non-Patent Literature 2), sulfuric-acid-based substances (e.g., sodium sulfite) (Non-Patent Literature 3), and ascorbic acids (Patent Literature 2).
However, the above substances are compounds that need to be handled with care during manufacturing or use because of risk of ignition. In addition, they are expensive and problematic in terms of biological safety. Further, when they are used for foods, they negatively influence the flavors of foods. Thus, they do not always satisfy requirements. Therefore, in many cases, it is necessary to carry out a step of separating or removing such reducing agents and by-products thereof from reduced coenzyme Q10 after the end of a reduction reaction. Accordingly, there has been a strong need for a substance that can be used after a reduction reaction per se as reducing agent or by-product thereof effective for stabilization of reduced coenzyme Q10, and can be applied to foods (e.g., health foods), supplements, and the like.
As an aside, some substances effective for reduction of oxidized coenzyme Q10 can be used as substances effective for stabilization of reduced coenzyme Q10; i.e., as antioxidants. It is known that reduced coenzyme Q10 is readily oxidized by oxygen in the air so as to result in oxidized coenzyme Q10. Therefore, a method for stabilizing reduced coenzyme Q10 by protecting a preparation of reduced coenzyme Q10 or a compound drug containing reduced coenzyme Q10 from oxidation is very important.
Known examples of compounds effective for stabilization of reduced coenzyme Q10 include citric acid and ascorbic acids (Patent Literature 3). In addition, it is known that hydrocarbons, fatty acid esters, ethers, and nitriles are preferable as solvents for stabilizing reduced coenzyme Q10 (Patent Literature 4).
However, the above substances and solvents effective for stabilization of reduced coenzyme Q10 and by-products thereof do not necessarily meet requirements suitably since they may be problematic in terms of biological safety, negatively influence the flavors of foods when used for foods, and be expensive. In some cases, further stabilization of reduced coenzyme Q10 is needed.
In view of the background above, a substance which can be used to effectively reduce oxidized coenzyme Q10 and/or stabilize reduced coenzyme Q10, and which is inexpensive, functional, and unlikely to be dangerous when handled during manufacturing has strongly been needed.